Stand-alone fusion cages have become popular in the field of spinal fusion surgery because they can be used without the need for posterior fixation. In general, at least two fixation devices (such as screws or nails) are passed at an angle through the anterior wall of the stand-alone cage and into the adjacent vertebral bodies, thereby securing the stand-alone cage to the adjacent vertebral bodies.
However, one challenge with these devices is that the need to pass the fixation device at an angle to the cage requires that the inserter also be disposed at an angle, thereby complicating the surgery. In some fusion cases using a stand alone cage, the surgeon must insert the fixation device into the cage at a sharp angle through sometimes challenging approaches. This can be especially difficult for the cervical spine, as the surgeon needs to either deliver the screw down into the inferior vertebral body but may be obstructed by the patient's chin, or deliver the screw up into the superior vertebral body but may be obstructed by the patient's sternum.
Therefore, there is a need to provide a stand alone fusion assembly that can be secured to the adjacent vertebral bodies via an in-line approach that is substantially perpendicular to the anterior wall of the stand alone cage.
U.S. Pat. No. 6,551,322 (Lieberman) discloses using a double helix as a fusion cage.
A number of investigators have attempted to solve the above-stated problem by in-line inserting a helical element into the in-situ cage. See for example, US 2002-0177898 (Crozet) U.S. Pat. Nos. 7,056,341; 6,210,442 (Wing); WO 00/16711 (Meriwether), and WO2010/028056 (Synthes). However, each of these solutions largely involves passing the helical element substantially through the graft-containing region of the cage, thereby reducing the space in the cage available for the critical bone graft.
Although the helix of FIG. 23 of Crozet does not enter the graft space, the height of Crozet's helix (a screw) would need to be only slightly greater than the height of the cage in order to avoid the hitting anterior lips of the adjacent vertebral bodies during insertion. Also, screw backout could be an issue.
Moreover, in each of these prior art solutions, the longitudinal axis of the helix is substantially parallel to the upper and lower surfaces of the cage.